When a charged particle strikes a surface, the particle may be scattered back (possibly losing some energy to the surface); induce the emission of a secondary particle (either ion or electron), or cause the release of a photon from the surface. From these outgoing (hereafter secondary) particles, characteristics of the surface can be determined, for example physical structure or material composition. The secondary particles can also, in turn, strike other surfaces inside an experiment, leading to tertiary emission and so forth. Since in many measurements, only the first surface is of interest, emission from other surfaces constitutes an unwanted background to the measurement.
As a specific example, the composition of a surface can be determined by studying Auger emitted electrons. Auger electrons can be observed by looking at the energy spectrum of the particles leaving the surface struck by a primary beam. The energy spectrum is measured by an energy dispersive analyzer: for example a hemispherical or cylindrical mirror analyzers or magnetic sector, to name a few. These devices observe a window of energies and exit angles from the surface. However, electrons from other surfaces outside or sometime inside the analyzer can be generated and reach the detector plane of the analyzer.